In recent years, there have been required a fertilizer, an agricultural chemical and a method for using them, which are of an advanced labor-saving type and high in efficiency, in view of; environmental impact of the leaching of components released from a particulate fertilizer, a particulate agricultural chemical or the like; labor-saving associated with aging farmers; and the like. By way of the above background, various fertilizers and agricultural chemicals of a controlled-release type have been proposed and applied to practical use.
Coated particles are formed by coating the surface of the particulate fertilizer or the particulate agricultural chemical with an organic or an inorganic coating material thereby controlling release of its interior components. Particularly, coated particles formed by using an organic coating material such as resin are superior in release-controlling function, so that such a type occupies the mainstream of the coated fertilizer and the coated agricultural chemical.
Resin used as the coating material are various but a thermosetting resin such as a urethane resin is widely applied because of its strength as a coating, good resistance to water, easiness of release-property control, applicability without solvent and the like.
In patent document 1, there is disclosed a coated granulated fertilizer formed by coating a granulated fertilizer with a reaction product of polyhydroxyl compound constituted of one kind of wool grease recovered in the case of washing wool, lanolin, lanolin fatty acid and lanolin alcohol or the mixture thereof and polyisocyanate. It is proposed to control the release by repeating a coating step, an organic-solvent spraying step and an evaporation step.
In patent document 2, there is disclosed a process for manufacturing membrane-covered fertilizer granules by applying layers of a polyisocyanate and a polyol, and more specifically, a process for manufacturing fertilizer granules to which coating materials are applied at such a ratio that each of the layers has a thickness of 10 to 30 μm, preferably 15 to 20 μm. It is proposed to control the release by repeating resin application, mixture, amine application and aeration.
Additionally, in patent document 3, there is disclosed a method for producing a granular coated fertilizer coated with a thermosetting resin, in which the following steps (1) to (5) are carried out in the order thereby producing a granular fertilizer free from defect in application: (1) a step of bringing the granular fertilizer into a rolling state; (2) a step of adding an uncured liquid thermosetting resin to the granular fertilizer which is under the rolling state, in an amount capable of providing 1 to 10 μm film thickness; (3) a step of coating the surface of each of fertilizer granules with the uncured thermosetting resin while keeping the rolling state of the granular fertilizer; (4) a step of curing the uncured thermosetting resin while keeping the rolling state of the granular fertilizer; and (5) a step of repeating the steps (1) to (4) one or more times. Incidentally, this patent document discloses that the film thickness is required to be 10 μm or lower, since aggregated granules consisting of myriad granules and formed such that the resin is so increased in adhesion property as to adhere the fertilizer granules to each other damages the surface of the resin.
Furthermore, there is set forth in patent document 4 a method for producing polyurethane encapsulated, sustained release fertilizer particles, including (1) a step of applying an isocyanate-reactive component containing a polyesterether polyol to fertilizer particles to form coated fertilizer particles and (2) a step of applying a polyisocyanate component onto the coated fertilizer particles from the step (1) to form polyurethane encapsulated fertilizer particles. These steps may be continuously repeated a required number of times in the use of the polyurethane encapsulated fertilizer particles of the step (2) instead of the fertilizer particles of the step (1) so that: polyurethane encapsulated fertilizer particles containing about 2 to 20 weight % of polyurethane relative to the total weight of an encapsulated fertilizer composition is formed; and polyurethane constituting the polyurethane encapsulated fertilizer particles absorb a water content of less than 1 weight % relative to the total weight of the polyurethane encapsulated fertilizer particles.